Mobile devices such as cell phones and PDAs (Personal Digital Assistants) have never been smaller and more functional. A variety of mount structures such as BGA (Ball Grid Array), and CSP (Chip Scale Package) are available as a mount technology for accommodating such advancements. Mobile devices are prone to mechanical load such as dropping impact. A QFP (Quad Flat Package) absorbs such impact with its lead portions. However, BGA and CSP do not have leads that relieve impact, and it is important to provide reliability against impact resistance in these structures.
Common Pb eutectic solders have a melting point of 183° C. Sn—Ag—Cu-based solders, a typical example of modern lead-free solders, have melting points about 30° C. higher than the melting point of Pb eutectic solders. This has raised the reflow furnace profile temperature to as high as 220 to 260° C. For mounting of components having weak high-temperature resistance to a circuit beard, such components are separately joined in a separate step by spot soldering. This has posed a serious drawback in productivity.
Under these circumstances, low-melting-point Pb-free solders of Sn—Sn, Sn—Ag—In, or Sn—Bi have been used lately to overcome the disadvantageous high melting points of Sn—Ag—Cu-based solders (hereinafter, “SAC solder”). However, a BGA connection using Sn—Zn—, Sn—Ag—In—, or Sn—Bi-based solders has not been fully established with regard to the interconnection reliability of the solder joint, particularly reliability against impact resistance.